SHANGHAI (AFP) Newly crowned golf world number one Lee Westwood said Wednesday he was rusty due to a lingering calf injury and will have to fight to keep his top ranking with other stars in striking distance. It is one thing getting there, but its another thing staying there. Its obviously very close at the moment, the Englishman told reporters in Shanghai where the seven-million-dollar HSBC Champions tees off on Thursday. Its going to be tough this week. I have to play well, which may not be easy because Im bound to be a bit rusty because Ive not played a lot just recently. So it should be a good week for everybody. Westwood dethroned Tiger Woods as world number one earlier this week, ending the American superstars latest five-year spell at the top. The ranking is based on performance over a sustained period. But he will face the strongest field in the six-year history of the HSBC Champions, which includes eight of the worlds top 10 including defending champion Phil Mickelson of the United States. Woods, Mickelson and Germanys Martin Kaymer all have a shot at wresting the top spot away from Westwood in Shanghai. Westwood was key to Europes Ryder Cup victory over America last month but has otherwise played relatively sparingly due to his calf injury, which was taking a lot longer to recover than he anticipated. Its still not right. But I was getting bored of sitting at home doing nothing and all of the people Ive seen have said I cant do any more damage to it, so to come out and the walking around may do it a little bit of good, he said. I do feel very rusty and havent played a lot, and this part of the year is really just a gauge towards being right for the start of next season. The 37-year-old conceded wistfully that even if he retains the top rank, he will ultimately be judged by his failure so far to win one of the four Major tournaments. I suppose getting to world number one, you look at the stats and the previous world number ones and that they have all won major championships. It would be nice to join that club as well, he said. You know, all I can do is keep putting myself in position and giving myself chances at winning, which I have done over the last couple of years.